Deformable mirrors have long been used as the correction element within adaptive optics systems. Such mirrors are typically: solid faceplate structures, where the faceplate is typically a reflective material such as a thin sheet of glass; or MEMS structures where the correction is applied by a plurality of small segmented mirrored surfaces; or membrane mirrors where the reflective surface is fabricated from a flexible polymer film, typically with a nominal radius of curvature bias imparted on the mirror surface.
Solid faceplate structures are best suited for high vibration environments or environments where high optical irradiances are encountered. MEMS mirrors offer a great deal of versatility, but are unsuitable for high power environments and are not well matched to some types of wavefront sensors. Membrane mirrors are inexpensive. but unsuitable for high power or high vibration environments. Furthermore, the base curvature in the mirror needs to be considered in the optical design, as base curvature will produce field angle-dependent aberrations.
Deformable mirrors function on the principle of having a thin surface that can be deformed to produce the conjugate aberration to the one measured by an associated wavefront sensor and which is incident on the deformable mirror. The deformation is effected by modulating the force applied to the mirror surface by a series of actuators. Most commonly, these are piezo-electric stacks for solid faceplate structures, although different mirror types will take advantage of different physical phenomena to create the localized deformations (e.g. electrostatic forces, bimorphic structures, etc. The actuator patterns may be rectilinear or arranged in other patterns, such as those describing various Zemike modes.
However constructed, prior art deformable mirrors suffer from one common limitation: actuator stroke is typically limited to a maximum of approximately 10 microns. Each actuator has a maximum slope and there is often some cross-talk between actuators. The cross-talk results in an inability to get full, independent motion from each actuator. Furthermore, because of the stroke limitations, the amount of aberration that can be corrected is limited. In most atmospheric aberration scenarios, the strengths of the various aberrations form an approximately geometric progression starting with tilt and progressing through the higher order aberrations (e.g. defocus, spherical, astigmatism, and coma). Most importantly, tilt can strongly dominate other aberrations. Thus, the stroke required to correct the tilt can leave little stroke left for correcting the higher order aberrations, as illustrated in FIGS. 1-3.
Known prior art includes: (1) U.S. Pat. No. 7,638,768, “Laser Wavefront Characterization”, L. J. Otten, et al.; (2) U.S. Pat. No. 8,009,280, “Wavefront Characterization and Correction”. G. R. Erry, et al.; and (3) U.S. Pat. No. 8,322,870, “Fast Steering, Deformable Mirror System and Method for Manufacturing the Same,” Kirk A. Miller.